One optical method for measuring fluid velocity is known in the art as “laser-two-focus” (L2F) velocimetry. L2F velocimetry is based on detection of light pulses caused by particles crossing two focused laser beams, and determining the average lapse time it takes for particles to move with the fluid from one focused beam to the other. The flow velocity is calculated by dividing the spacing between two laser beams by the lapse time. The flow velocity can be measured using L2F velocimetry to an accuracy of within 0.01% based on accurate beam focusing and fast digital signal processing. For this reason, L2F velocimetry is particularly suitable for study of complex flow of gases and transparent liquids. Other applications include measurement of gases in the pipes at very low (slower than 0.1 m/s) to very high (faster than 100 m/s) speeds. Such high range is necessary for measurement of fluctuated gases such as flare gases in large pipes.
There exist L2F velocimeters for measuring complex fluid flow in small pipes and channels such as ones used in turbomachinery (e.g., U.S. Pat. No. 3,941,477 to Schodl and No. 4,707,130 to Hofmann et al.). Such systems include a housing with an optical illumination system configured to direct two laser sheets through the flow, and an optical collection system to collect light which is back-scattered light by particles in the flow and to focus it into two photodetectors. The optical axis of the collection system is parallel to the sheets of light. The illumination system sends the light through a central area of the collection system, which collects the light back-scattered from the particles received in a peripheral area.
Referring to Hoffmann et al., which discloses an example optical arrangement of this first type of prior art, the optical arrangement includes a laser and a splitter which create two parallel beams, and a lens and two mirrors configured to direct the beams through an optical cell having a convex lens and a concave lens which focus the beams on two test points. Light from the two beams is back scattered from the test points through the optical cell and another lens and directed onto photomultipliers through a shutter.
One shortcoming of such systems is that the intensity of back-scattered light is much lower than of forward-scattered light. This reduces the signal-to-noise ratio to such a degree that such systems generally require particle seeding (adding particles to the flow) to operate. Particle seeding, however, is not allowed in many petrochemical applications such as natural gas pipelines, flare stacks and venting pipes.
Such systems also do not allow for the cost effective design of compact velocimeters to be used in large pipes, because if the testing area is placed far from the housing, there must be a proportional increase in the size of the aperture of the optical collection system. Large optical systems increase the cost of the velocimeter.
Another type of prior art optical gas flow meter which is based on a L2F velocimeter comprises a housing rigidly mounted to a plate providing direct access to the fluid flow in a pipe (e.g., U.S. Pat. No. 6,128,072 to Kiel et al.). The optical collection system detects the forward-scattered light but it is displaced at a certain angle from the optical axis of the illumination system in order to avoid capturing the unscattered light.
Referring to Kiel et al., which discloses an example optical arrangement of this second type of prior art, the optical arrangement is based on a plate with a central hole that preferably matches the inside diameter of the pipe through which the fluid flows. An optical fiber mounted on the plate terminates at a collimator to direct light from the fiber to a prism which splits the light into two beams. The beams are reflected by a mirror and focused by a cylindrical lens to create two parallel beams through a measurement volume. Light from the beams is scattered by particles passing through the measurement volume and a portion of the scattered light is collected by a refractive doublet and focused to an image point. The light is preferably collected at an angle in the range of 5 to 25 degrees from the direction of the parallel beams.
One drawback of this second type of prior art velocimeter is that it is not suitable for pipes of various diameters, since the plate and optical system must be designed individually for each pipe size. Also, the off-axis location of the optical collection system reduces collection efficiency of the collection system because it only accepts the scattered light in a limited solid angle. Furthermore, it is not practical to insert this second type of prior art velocimeter into very large pipelines.
Another disadvantage of the second type of prior art systems is that optical meters installed in pipelines can be affected by window fouling. Accordingly, it is necessary to have a housing which will allow removing the optics for cleaning purpose without depressurizing the pipe. Removing the plate according to the above art requires a complex mechanical setup which with combination of complex high-pressure fiber optic feed-through makes the meter expensive.
Prior art L2F velocimeters can be accurate. However, they tend to determine the fluid velocity in a limited testing area where laser beams are focused. This makes them vulnerable to differences in velocity across the flow profile. Kiel et al. discloses a multi-point L2F velocimeter wherein one testing area is located in the center of the pipe and a number of testing areas are located a quarter-radius distance from the wall. The quarter-radius location is less susceptible to flow profile, according to fluid dynamic calculations. Multi-point L2F velocimeters, however, require complex optical systems to create multiple testing zones across the flow profile.
Therefore, there is a need for optical velocimeters which do not require particle seeding and which are suitable for fluid velocity measurement in pipes of various diameters. There is also a need for velocimeters having housings that are easily removable from fluid carrying pipes for cleaning purposes and maintenance.